Intraprosthetic dislocation: a potentially serious complication of dual mobility acetabular cups.

Though dual mobility cups have gained growing popularity as a solution for instability in total hip replacements, these promising devices are subject to a specific implant failure mode, named intraprosthetic dislocation. We present the case of a patient sustaining such an adverse event. The planned revision surgery was postponed 12 months due to a severe heart condition, allowing a rare opportunity to document the natural history of this unusual complication. The small femoral head was found dislodged in the superior part of the metallic shell and had remarkably lost its sphericity. Severe metallic debris and granuloma were found in the proximal femoral region, associated with major periprosthetic bony and soft tissue damage. Surgeons, radiologists, and general practitioners should be aware of this specific complication, its incidence (almost 5%) and mechanisms (femoral neck to mobile polyethylene insert impingement, leading to rim fatigue and wear of the insert at the capturing area). Diagnosis is mainly based on anteroposterior and modified Lowenstein lateral radiographs of the hip, as an eccentric position of the small femoral head, lying against the concave inner surface of the shell. Prompt component revision should be planned, since delayed management could lead to severe irretrievable damages.